Answer: 2.
I feel like this is sort of a trick question if you look at in a real world perspective. Mathematically speaking, if you use the simplest way to solve for x, the answer would be 2. However, in the real world, this question doesn't tell us what rate Dalton reads at. As in how many books he read per month. In the real world, he could have read those 18 books over any period of time. He may have read them in under a month, he may have read them throughout 100 months or more. There's not enough information to accurately say how long he has been a member of the book club.
